Uterine Tumors

By Malcolm Weir, DVM, MSc, MPH; Debbie Stoewen DVM, MSW, RSW, PhD; Christopher Pinard, DVM

What is a uterine tumor?

A uterine tumor is a type of tumor that develops from the uncontrolled disordered growth of one of the cell types found within the uterus. The uterus is comprised of many types of cells and layers of tissue, including smooth muscle, epithelium (skin cells), and glandular tissue. Tumors may arise from any one of these cell types.


Leiomyomas (benign, non-cancerous tumors) and leiomyosarcomas (cancerous tumors) arise from muscle cells, squamous cell carcinomas arise from epithelial (skin) cells, and adenocarcinomas arise from the glands found within the uterus.

Because many dogs and cats in North America undergo ovariohysterectomy (spay), the incidence of uterine cancer is quite low. For pets that are intact (not spayed), the occurrence of malignant (or cancerous) uterine tumors is rare in dogs and even more rare in cats.

85-90% of uterine tumors in dogs are leiomyomas; benign (non-cancerous) tumors that develop from the smooth muscle cells of the uterus. Cats, on the other hand, most commonly develop endometrial adenocarcinomas, malignant tumors that develop from the glands of the uterus.

What causes this cancer?

The reasons why a particular pet may develop these, or any other kinds of tumors is not straightforward. Very few cancers have a single known cause. Most seem to be caused by a complex mix of risk factors, some environmental and some genetic or hereditary. In the case of uterine cancer, there is no known cause, although there may be a possible link to the use of hormone therapy and the development of uterine carcinoma.

What are the signs that my pet has a uterine tumor?

The signs of a uterine tumor vary depending on the type of tumor and what areas of the body it may affect. Signs that your pet has a uterine tumor may include a distended (uncomfortable) abdomen, vaginal discharge, development of pyometra (pus in the uterus), constipation, difficulty urinating, lethargy, lack of appetite, vomiting, and weight loss. Signs may also include increased drinking and urination.

How is this cancer diagnosed?

These tumors are generally difficult to diagnose initially and may be found with a physical exam or routine x-ray screen.  On physical examination, your veterinarian may be suspicious of this condition when they palpate (feel) a noticeable mass in the abdomen.

Some tumors will produce hormones causing your pet’s vulva to look larger than normal. Increased levels of the hormone estrogen may also be identified on specific hormonal blood tests. In pets with uterine tumors, clinical signs such as vaginal discharge, constipation, difficulty urinating, vomiting, and a distended abdomen may be observed.

"By far, uterine tumors are most commonly diagnosed by abdominal ultrasound..."

By far, uterine tumors are most commonly diagnosed by abdominal ultrasound (a mass is observed) or during a spay procedure. If a spay procedure is performed, any abnormal tissues will be submitted to a pathologist for review, called histopathology. The pathologist will examine the tissues under a microscope and determine the type of tumor and if it is benign or cancerous.

How does this cancer typically progress?

In the majority of cases, uterine tumors are more locally aggressive (meaning they penetrate local tissues) with a low rate of metastasis (spread to other parts of the body). Your veterinarian may recommend full staging (searching for potential spread to other locations in the body) prior to surgery. This may include bloodwork, urinalysis, X-rays of the lungs, and possibly an abdominal ultrasound. If any abnormalities are discovered, these areas may be sampled prior to, or at the time of, surgery to determine if the cancer has spread elsewhere.

What are the treatments for this type of tumor?

Ovariohysterectomy (spay) is by far the most commonly pursued treatment option. In the case of benign or locally growing tumors, it is the treatment of choice as the tumors are completely removed.

If staging and other diagnostic tests have indicated metastasis (spread to other areas in the body), then a spay procedure may still be performed, along with removal of other affected tissues or lymph nodes. Follow-up with chemotherapy may be pursued, though there is limited knowledge regarding its success.

Is there anything else I should know?

These types of tumors are quite rare given that the majority of pet owners in North America spay their pet dogs and cats. Breeding animals, given repeated estrus cycles and pregnancies, with exposure to fluctuating sex hormones over an extended period of time, may be more prone to developing these tumors. Regardless, the appropriate time to spay your pet, as well as the potential implications of not spaying your pet should be discussed with your veterinarian early in your ownership. These cancers are treatable but spaying typically has to be pursued.

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